If Earth's past climate cycles are any indication, temperatures could be significantly hotter by the end of the century than current climate models predict. New research suggests that current atmospheric models underestimate future global warming. Scientists say the estimates don't account for soil decomposition and other natural processes that are expected to escalate in response to ongoing warming, thus amplifying greenhouse gas production.

Currently, the Intergovernmental Panel on Climate Change estimates that the global average temperature could increase as much as 5.8 degrees Celsius by the end of the 21st century. But these estimates don't factor in some feedback mechanisms that may be triggered by rising temperatures. For instance, accelerated decomposition in soils and changes in ocean chemistry may add considerably to greenhouse gases and further intensify warming. Two studies published 26 May in Geophysical Research Letters attempt to translate these potential impacts into hard numbers.

In the first study, biogeochemists Margaret Torn of Lawrence Berkeley National Laboratory in California and John Harte of the University of California, Berkeley, used Antarctic ice cores to estimate the amount of greenhouse gases released into the atmosphere over the past 420,000 years, allowing them to predict the impact of future climate-CO2 feedbacks. Combining these estimates with standard assumptions from climate models, they calculated the amplification in global temperatures attributable to greenhouse gas feedback. They found that a doubling of current CO2 levels would boost temperatures by 1.6 to 6 degrees Celsius, and by 2100 the gain could be as much as 7.7 degrees C.

Torn notes that many feedback mechanisms remain poorly understood, and uncertainties abound in trying to predict their effects on climate. But she believes the findings indicate "that we will experience more severe, not less severe, climate change than is currently forecast."

In the second study, climatologist Marten Scheffer of Wageningen University in the Netherlands and colleagues also examined data from polar ice cores and reconstructed temperatures during the ice age that lasted from about 1550 to 1850. Using somewhat different methods from Torn and Harte, they found that warming due to human activities could heighten temperatures by 1.7 to 8.0 degrees Celsius over the coming century.

Caspar Ammann, a climate modeler with the University Corporation for Atmospheric Research in Boulder, Colorado, however, notes that climate models don’t predict greenhouse gases per se, but rather they predict the outcome of different climate scenarios: How high will temperatures rise if CO2 doubles, for instance. These scenarios do incorporate different types of feedbacks, including advances in technology and public policy, he says. As climate feedbacks from warming soils and oceans--both huge carbon reservoirs--become better understood, the models will become more precise, says Ammann.